A gypsum ceiling design is not complete until the perimeter is sealed. The structural junction where the wall meets the ceiling sheet is often rough, showing gaps or metal framing.
Installing a cornice covers this joint and provides the final aesthetic polish to the room. While many homeowners focus on central lighting bulkheads, the quality of the cornice installation determines whether the finish looks professional or amateur.
Planning Your Gypsum Ceiling Finish
Successful installation begins with accurate material selection and quantification. Purchasing the wrong size or underestimating the required length leads to visible joints and work stoppages.
Choosing the Right Cornice Profile
Cornice profiles in Kenya range from simple 75mm coves to elaborate 200mm floral designs. The correct choice depends on your ceiling height, not just personal taste. Installing a large, heavy cornice in a standard apartment with a 2.4-metre ceiling will make the room feel cramped and the ceiling lower.
For standard room heights (2.4m – 2.6m), a 100mm to 125mm plain or stepped profile is proportional. Reserve large, decorative profiles for maisonettes or commercial spaces with clearance above 3 metres.
Estimating Materials and Cost
To calculate the material required, measure the total perimeter of the room in metres. Divide this number by the length of the standard cornice strip available from your supplier (typically 2.4 metres or 3.0 metres).
You must add 10% to 15% to this total for waste. Cutting corners (mitres) generates offcuts that often cannot be reused elsewhere. If you have many corners or pillars in the room, increase the waste factor to 20% to avoid running short.
Essential Tools and Adhesives
Achieving seamless joints requires specific tools. Attempting to cut corners freehand or using the wrong bonding agent will result in cracks and mismatched angles.
The Mechanical Toolkit
You cannot cut accurate internal and external corners without a guide. A mitre box is the most essential tool for this job. It holds the cornice at the correct angle while you cut. Use a fine-tooth panel saw or a dedicated gypsum saw to ensure a clean cut without tearing the paper facing of the cornice.
You will also need a sponge and a bucket of clean water to wipe away excess adhesive before it hardens.
The Bonding Agent: Plaster Powder vs. Acrylic Adhesive
The traditional installation method involves mixing casting plaster (cornice cement) with water in a bucket. While cheap, this method is messy, inconsistent, and creates significant waste as the mix hardens rapidly in the bucket. It also tends to drop plaster lumps onto the floor.
The modern, professional alternative is using a gun-applied acrylic adhesive, such as DLG A6755. This adhesive comes in a cartridge and applies a consistent, clean bead directly to the cornice edges. It offers high initial grab, meaning the cornice sticks immediately without sliding.
This method eliminates the mess of mixing powder and is the superior choice for occupied homes where protecting the flooring is a priority.
The Hard Part: Cutting Internal and External Corners
Cutting the corners is the most challenging aspect of cornice installation. A standard 90-degree cut will not work because the cornice sits at an angle across the junction of the wall and ceiling. You must cut a compound angle to make the two pieces meet perfectly.
Understanding the Mitre Box Technique
The secret to a perfect cut is positioning the cornice correctly in the mitre box. You must place the cornice in the box upside down. The edge of the cornice that will touch the ceiling must sit on the bottom of the box, and the edge that will touch the wall must rest against the side of the box facing you. This position mimics the sprung angle of installation. Once placed, hold it firmly; any movement during sawing will result in a gap at the joint.
Cutting an Internal Corner (Wall to Wall)
An internal corner is where two walls meet inside the room. Measure the length of the wall and mark this measurement on the bottom edge of the cornice (the wall edge).
When cutting the left side of the corner, cut from right to left in the mitre box. When cutting the right side, cut from left to right. The back of the cornice (the part touching the wall) will be longer than the front face. Always test the fit of your two cut pieces on the ground before applying adhesive.
Cutting an External Corner (Chimney Breast or Pillar)
An external corner points outwards into the room, such as around a structural column or a chimney breast. These cuts are the reverse of internal corners.
The front face of the cornice will be longer than the back edge that touches the wall. You must allow the cornice to extend past the corner of the wall before the cut begins. These joints are highly visible, so precision here is mandatory for a clean finish.
Installation: Fixing the Cornice to the Wall
Once your pieces are cut and dry-fitted, the fixation process begins. Adhesion fails when surfaces are dusty or sealed with glossy paint.
Surface Preparation and Keying
Gypsum cornices bond best to raw plaster or matte surfaces. If you are installing over existing silk or gloss paint, the adhesive may peel off the paint layer. You must “key” the surface.
Use a utility knife or the edge of a scraper to score crisscross lines into the paint along the top of the wall and the edge of the ceiling. This scoring penetrates the paint layer and gives the adhesive a rough texture to grip. Wipe away all dust with a damp cloth immediately before installation.
Applying the Adhesive (The Clean Fix)
Using a cartridge-based adhesive like the DLG A6755 simplifies this step significantly. Load the cartridge into a standard caulking gun and cut the nozzle to create a 5mm opening.
Apply a continuous bead of adhesive along the top flat edge (ceiling side) and the bottom flat edge (wall side) of the cornice. Do not put adhesive in the middle hollow section.
Press the cornice firmly into the wall-ceiling junction. Apply slight pressure and wiggle the piece up and down by a few millimetres.
This action breaks the surface tension and creates a suction effect, holding the heavy gypsum strip in place without the need for temporary nails or supports. Use a damp sponge to wipe away any adhesive that squeezes out from the edges immediately; dried acrylic is difficult to sand down later.
Finishing: Filling Gaps and Painting
Even with precise cutting, small gaps will appear between the cornice and the wall, or at the mitred corners. Filling these gaps is necessary to create the illusion that the cornice is a single, continuous cast. The material you choose for this stage determines whether your paint finish will look consistent or patchy.
Why You Must Use Paintable Sealant (Not Silicone)
A common mistake is filling these gaps with standard sanitary silicone. Silicone is hydrophobic, meaning it repels water-based liquids, including emulsion paint. If you use silicone, the paint will bead up and refuse to stick, leaving an ugly, unpainted line around your ceiling that is impossible to cover.
You must use a dedicated Acrylic Sealant (often called decorators caulk). The DLG Acrylic Sealant is formulated to remain flexible like silicone but has a surface structure that accepts paint readily. Run a thin bead along the top and bottom edges and smooth it with a wet finger to blend the cornice into the wall.
Painting the Cornice
Gypsum is highly porous and has strong suction. If you apply a standard topcoat emulsion directly to raw gypsum, the material will suck the moisture out of the paint too quickly, causing it to flake or appear patchy. You should first apply a “mist coat” (a mixture of water and emulsion) or a dedicated drywall primer.
This seals the surface. Once the primer is dry, you can apply two coats of high-quality vinyl matt or soft sheen paint to match or contrast with your ceiling.
Frequently Asked Questions
What is the price of a gypsum ceiling in Kenya?
The cost of a complete gypsum ceiling installation, including metal framing, boarding, and finishing, typically ranges from KES 2,200 to KES 3,500 per square metre. This price varies depending on the complexity of the bulkheads and the quality of the paint finish selected.
Why is my cornice cracking at the joints?
Cracks at the joints usually indicate movement or poor adhesion. If the building is new, settlement cracks are common. However, if the adhesive was applied only to the wall and not the ceiling (or vice versa), the cornice creates a weak bridge that snaps when the house expands in the heat. Using a flexible acrylic adhesive helps absorb this thermal movement.
Can I install cornices on a concrete ceiling?
Yes, but you cannot glue directly to smooth concrete. The adhesive will slide off. You must first “hack” the concrete surface with a hammer and chisel to create roughness, or apply a bonding agent (PVA bond) to provide a grip for the cornice adhesive.
